The present invention relates generally to heat sink arrangements for use in assemblies employing multiple semiconductor devices.
Because heat is one of the most common causes of semiconductor failure, in almost every application of semiconductor devices where any significant amount of power is being controlled, the semiconductors are mounted on or in a heat exchange relationship with some form of heat sink. These heat sinks often also form a part of the electrical circuitry. This is particularly true in the case of the flat package type of semiconductor device presently in widespread use. In this type of device, which is variously called a "pancake," "disc," "hockey puck" and one form of which sold by General Electric Company under the trademark "PRESS-PAK," the actual semiconductor material is retained between two flat conducting plates which constitute the anode and the cathode terminals. If the semiconductor device is other than a diode having only two electrodes, for example a thyristor, then the additional electrodes, such as a gate electrode, are elsewhere located for electrical connection. Because the heat sink is a part of the electrical circuitry, restrictions on construction are inherent. That is, because certain electrodes cannot be electrically connected, electrical insulation is required between various portions of the heat sink portion of the total assembly.
In many applications where a plurality of semiconductor devices collectively perform a unitary function, only a portion of the SCR devices are active or operational at any one particular instant in time. One example of such an operational package is the three phase, full wave, reversing bridge for driving a d.c. motor bidirectionally from an a.c. source. These bridges have two groups of semiconductors, one group for providing motor operation in the forward direction and a second group for providing motor operation in the reverse direction. Typically, when the bridges are of the phase control type, six thyristors are provided for forward motor operation and six for reverse operation. Customarily, the bridge is modularized and has the six forward thyristors connected as one unit and the six reverse thyristors as the second unit. Thus, if only a single direction drive is required, only one module is required.
In addition to the semiconductor devices and appropriate heat sinks, the total assembly usually also includes what are known as "snubber" circuits. Such circuits are well known and the snubber circuit is often no more than an RC network (e.g., a simple resistor and capacitor in series) which is connected in parallel with the semiconductor device. The primary purpose of the snubber circuit is one of protection. That is, the snubber circuit protects the semiconductor device from transients which might damage the device or cause a false firing thereof.
As earlier mentioned, one of the primary concerns with respect to semiconductor power apparatus is heat and thus the rating of any bridge assembly or similar type circuit is a function not only of the semiconductor devices themselves and their associated heat sinks but also of the manner in which the semiconductors are to be operated (i.e., the duty cycle). With a given semiconductor size, an increase in the duty cycle of the bridge will require greater heat dissipation capacity of the sink portion of the total assembly. In the prior art, once the design and cooling method were determined, increased capacity was usually achieved by increasing the heat sink size with the attendant increase in costs for material and packaging. In addition, the prior art requires individual snubber circuits for each of the semiconductor devices which also tends to raise the price of the overall apparatus or assembly.